/**
 *
 * @param {AdmitHub.Model.UserSearches} userSearch the userSearch object containing the query to run
 * @returns {String} an array of user ids matching the provided query
 */
const performPseudoAggregationQuery = function (userSearch) {

  const groupMatchSets = [];

  // For each or-group of the search
  for (let searchGroup of userSearch.orGroups) {
    let termMatchSets = [];

    // For each search term in this or-group
    for (let searchTerm of searchGroup.searchTerms) {

      // Determine collection to search on and the field on that collection that ids a user
      const collectionClass = AdmitHub.Collection[searchTerm.searchCollection];
      const idField = (searchTerm.searchCollection === 'Users') ? '_id' : 'userId';

      // Perform query
      const termMatchSet = collectionClass.find(
        {[searchTerm.field]: {[searchTerm.operator]: searchTerm.value}},
        {fields: {[idField]: 1}}).map(u => u[idField]);

      termMatchSets.push(termMatchSet);
    }

    // Intersect term match sets (perform AND within group)
    const termMatches = termMatchSets.reduce((memo, current) => {
      return new Set([...memo].filter(u => current.has(u)));
    });

    groupMatchSets.push(termMatches);
  }

  // Union group match sets (perform OR between groups)
  const allMatches = groupMatchSets.reduce((memo, current) => {
    return [...memo, ...current];
  });
  return [...(new Set(allMatches))];
};